A non-magnetic Fe probe of multiple magnetic phase transitions in RMn²Si_2-xGe_x, R = rare earth

Dilute ⁵⁷Fe Mössbauer-spectroscopy studies of RMn₂X₂ (X = Si and/or Ge, R = La, Ce, Pr, Nd, Sm, Eu and Gd) at 4.2 to 650 K yield the following results: Fe in RMn₂X₂ does not carry a magnetic moment. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields (∼ 100 kOe). The compounds LaMn₂Si₂, LaMn₂Ge₂, CeMn₂Ge₂, PrMn₂Ge₂, NdMn₂Ge₂ and SmMn₂Ge₂, known to be ferromagnets with T_c = 300-350 K, are antiferromagnetically ordered above their corresponding T_c. Ther T_N values extend from 385 K (SmMn₂Ge₂) to 470 K (LaMn₂Si₂). At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In RMn₂Si_2-xGe_x with intermediate x values, the Ge is much more dominant in determining the magnetic properties of the Mn sublattice. While PrMn₂Si₂ is an antiferromagnet (T_N = 365 K) and PrMn₂Ge₂ is a ferromagnet-antiferromagnet (T_c = 328 K, T_N = 415 K), we find that in PrMn₂SiGe, the magnetic behaviour is similar to that in pure PrMn₂Ge₂, T_c = 305 K and T_N = 395 K.